[Aavso-photometry] Landolt error bars

[Ben Davies]

arne wrote:
> Ben Davies wrote:
>> I want to get the U and  B error numbers for some of Arlo Landolt's 
>> standard fields and am wondering if the color error magnitudes given 
>> in his paper 
>> <http://articles.adsabs.harvard.edu//full/1983AJ.....88..439L/0000444.000.html> 
>> are just quadrature sums of the individual UBV errors.
>>
>> If they are, and I want to back out the U and B errors, do I need to 
>> first convert them to flux-like values (ie not logarithmic), do the 
>> subtraction and then reconvert to magnitudes?
>>
>> Also, in the paper, I notice that in numerous instances the (for 
>> example) reported (B-V) errors are smaller than the V errors.  How 
>> can that be?
>>
>> Can anyone get me pointed me in the right direction here?
>>
> Not quite sure what you mean, but I'll take a stab at it.
> The normal method of all-sky data reduction is to use transformation
> equations that give the V magnitude, plus a number of color indices.
> These color indices are of course formed by taking two instrumental
> magnitudes and subtracting them.  The formal error for an individual
> measurement set does depend on the Poisson error of each measure,
> plus lots of other terms like how well extinction was determined on
> a night, how well the transformation fits the standard stars observed
> on that night, etc.
>
> So the all-sky calibration is *not* done as U, B, V, R, I and then
> forming (U-B), (B-V) for the tables.  Instead, each color index has
> its own equation and transformation coefficient.  The error in the
> tables represents the error in the color indices themselves.  The
> reported color indices are the means from several nights, and the
> errors are the standard deviations from the mean across those nights.
> Once you do this, the Poisson error, etc. of the individual measure
> goes away since you are creating an empirical error across many
> nights of individual measures.
>
> You can have smaller error in a color index than in a magnitude
> because, to first order, the extinction and any transparency
> variations cancel out when you take the difference of two magnitudes,
> much like differential photometry between two stars.
> Arne
>
Arne,

Thanks for the explanation.  I didn't put the question very well.

The reason I want the Uerr and Berr numbers is that I am using the 
FITEXY routine in IDL to get a chi-square linear fit to model  color 
terms like B-b vs (B-V).  FITEXY will use the error bars to get a better 
fit if you have them.

I know these error numbers exist for UBRI , because I have them for a 
few fields.  I just don't know where or how to get others.  Maybe there 
is a text file somewhere?

Then, I am trying to figure out how would I construct the error bar for 
the B-b axis.  This should have been the quadrature question.

Ben